The present invention relates to an apparatus for recording a binary image, e.g. a line image, by scanning a light beam or beams, and more particularly to an apparatus for recording a binary image for use in manufacturing a printed wiring board with high accuracy.
An apparatus (such as the apparatus shown in FIG. 9) has been used to record a binary image. In the apparatus, a laser beam L.sub.0 emitted from a laser tube 91 is divided by a beam splitter 92 into a plurality of beams, e.g. twenty beams. (Only six beams are illustrated in the drawing for simplicity of illustration.) The beams are received by an acousto-optical modulator (hereinafter referred to as "AOM") 93, by which the beams are modulated according to image signals Is applied thereto. The "beams modulated" are focused on a photosensitive material 96 by an objective lens 94, the photosensitive material 96 being loaded on the peripheral surface of a recording cylinder 95. The recording cylinder 95 rotates in the direction denoted by an arrow X (hereinafter referred to as "main scanning direction"), while it is moved in the direction denoted by an arrow Y (hereinafter referred to as "subscanning direction"), whereby a desired binary image is recorded on the photosensitive material 96 by the laser beams.
To record of such a binary image as a circuit pattern for a printed wiring board, two factors are most important: (1) smoothness of the margin of the image and (2) high dimensional accuracy. Conventionally, the diameter of each of the recording beams is expanded considerably (relative to that of the corresponding pixel), to improve the smoothness of the margin. The recording beams are overlapped considerably to improve the smoothness of the margin of the image.
FIG. 10 is a graph showing a distribution of light quantity imparted onto the photosensitive material. As shown in the drawing, the quantity of light imparted by each of the respective recording beams represents a Gaussian distribution denoted by numerals W.sub.1 to W.sub.n. The distribution of light quantity as a whole is shown by a curve W.sub.s.
Different kinds of photosensitive materials have different inherent, the critical exposure, values, as denoted by numerals e.sub.0 to e.sub.2, according to the sensitivity of the particular photosensitive material. Therefore the recording width effected by the beams is variable, as shown by numerals d.sub.0 to d.sub.2, according to the critical exposure values inherent in the different kinds of photosensitive materials. Such variation in the recording width may be as great as one pixel. This creates a serious problem of accuracy. That is, an inaccurate circuit pattern may be created when the recording width is varied by as much as one pixel.
Particularly in manufacturing a printed wiring board, it is often the case that several circuit pattern images must be repeatedly on a photosensitive material with high accuracy, e.g. within the range of .+-.3.0 .mu.m. Accordingly, the recording width must be accurately controlled so that even different kinds of photosensitive materials can be recorded with a highly accurate binary image, e.g. within the range of .+-.3.0 .mu.m.